In the present paper, the phylogenetic relationships between genera, subfamilies and families of the Hemiuroidea are explored. Twelve new sequences of 28 rDNA and data taken from GenBank (NSBI) on 43 species affiliated to 34 genera were included in the analysis. Most of the hemiuroidean trematodes form two highly supported clades (A and B), which are sister groups to each other.Hemipera manterijoined withGonocercaspp. with moderate statistical support. This clade is basal relative to the clades A and B. Сlade A is polytomic and contains representatives of the families Accacoeliidae, Syncoeliidae, Didymozoidae, Hirudinellidae and Sclerodistomidae, and derogenid subfamilies Derogeninae and Halipeginae. At the same time, the Syncoeliidae, Hirudinellidae and Accacoeliidae form a well-supported monophyletic group. The phylogenetic relationship between Derogeninae and Halipeginae is poorly resolved. Сlade B unites the isoparorchiid, bunocotylid, lecithasterid and hemiurid trematodes. Our data re-establishes the family Bunocotylidae, which consists of two subfamilies, Opisthadeninae and Bunocotylinae, and theMachidatrema chilostoma+Hysterolecithoides frontilatusgroup. The Bunocotylidae is the sister group to the Hemiuridae + Lecithasteridae group and the Isoparorchiidae is a basal relative to the representatives of these three hemiuroid families.

The sensory organs in tegument of two trypanorhynchean species--Nybelinia surmenicola (plerocercoid) and adult Parachristianella sp. (Cestoda, Trypanorhyncha)--were studied with the aim of ultrastructural description and a comparative analysis. The Nybelinia surmenicola plerocercoid lacks papillae with sensory cilia on the bothria adhesive surface. We found an unciliated sensory organ within the median bothria fold. This unciliated free nerve ending contains the central electron-dense disc, three dense supporting rings, and broad root. The nerve ending locates in the basal matrix under the tegument. The tegument of N. surmenicola has a number of ultrastructural features which make it significantly different from other Trypanorhyncha: (i) the tegumental cytoplasm has a plicated constitution in a form of high apical and deep basal folds, (ii) numerous layers of the basal matrix are presented in the subtegument, and (iii) the squamiform and bristlelike microtriches N. surmenicola lack the base and the basal plate. In contrast, numerous ciliated and unciliated receptors were found in Parachristianella sp.: six types on the bothria and one type in the strobila tegument. Ultrastructural constitution of sensory organs in the form of ciliated free nerve endings as well as unciliated basal nerve endings of Parachristianella sp. has many common features inside Eucestoda. In comparison with other Trypanorhyncha, all Nybelinia species studied have less quantity of the bothrial sensory organs. This fact may reflect behavioral patterns of Nybelinia as well as phylogenetic position into Trypanorhyncha. Our observations of living animals conventionally demonstrate the ability of N. surmenicola plerocercoids to locomote in forward direction on the Petri dish surface. The participation of the bothrial microtriches in a parasite movement has been discussed.

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For the first time, the results of the study of the age and growth of blue hake Antimora rostrata in the waters of the Lazarev and Weddell seas (Antarctic) are presented. The longline catches were represented by fish from 42 to 69 cm in total length with weights between 420 and 2,900 g, and most individuals aged 25 to 27 years. A minimum age of 16 years was observed in a fish 47 cm long and weighing 450 g, while a maximum age of 35 years was recorded for an individual of 69 cm in length and 1,640 g in weight. The blue hake in the Lazarev and Weddell seas shows similar growth patterns to the fish from the Ross Sea and waters off Greenland.

The fine structure of the nervous system in plerocercoid Ligula intestinalis (Diphyllobothriidae) has been studied. The ultrastructure of the cerebral ganglion, the main nerve cords and peripheral nervous system has been described. The cerebral ganglion has complicated microanatomy. It consists of symmetric pair frontal and lateral lobes connected by the massive median commissure, and ventral azygos lobe, located in the center and on the ventral surface of the commissure. Two crossed ventral nerves are originated from the unpaired median (ventral) lobe. It was found, that the neurons in the lateral lobes and the commissure have asymmetrical position (localization neurons in the lateral lobes and the commissure has dorso-ventral polarity): large neurons (2040 µm) are concentrated on the dorsal surface of the cerebral ganglion. We found four types of neurons with different fine structure and size, peripheral neurosecretory cells and undifferentiated cells. Three types of synaptic contacts have revealed in neuropiles. Giant axon occurred as part of the main cord. On the surface membrane of the giant axon are numerous incoming synapses. The peripheral nervous system consists of numerous compact bunches of nerve processes taking place along the longitudinal and dorso-ventral muscle fibers. Also three types of sensory organs were found in the tegument: one type a ciliate and two types unciliate receptors. Comparative analysis of nervous system organization in representatives of the two orders Diphyllobothriidea and Bothriocephalidea showed that the cerebral ganglion L. intestinalis possesses bilateral symmetry, characterized by the presence of the ventral lobe, ventral nerves and the dorso-ventral polarity in the arrangement of neurons and exiting nerves. On the contrary, the cerebral ganglion of Triaenophorus nodulosus (Bothriocephalidea) has a double-beam symmetry: it has symmetrical lobes in ganglion with symmetrical dorsal and ventral nerves, lack of lobe in the median commissure, and send symmetrical dorso-ventral rootlets in the bothria. Our data confirm the belonging of the studied species to different groups and support the idea of separation Pseudophyllidea sensu lato into two new orders Diphyllobothriidea and Bothriocephallidea.

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A new species, Caudotestis dobrovolski n. sp., is described from Liparis sp. (Scorpaeniformes: Liparidae) and Eumicrotremus fedorovi Mandrytsa, 1991 (Scorpaeniformes: Cyclopteridae), caught in the Simushir Island area of the North Pacific. This species differs from six previously known congeners by the following combination of features: the excretory vesicle reaches to the anterior edge of the ventral sucker, uterus pretesticular, genital pore prebifurcal and median, and testes entire or with irregular outline, occasionally distinctly lobate. Phylogenetic analysis of 28S rRNA gene partial sequences suggests a sister position of Caudotestis Issaitschikov, 1928 to the genus Biospeedotrema Bray, Waeschenbach, Dyal, Littlewood, & Morand, 2014, and unites C. dobrovolski n. sp. + Biospeedotrema spp. with Zdzitowieckitrema incognitum Sokolov, Lebedeva, Gordeev, & Khasanov, 2019. The phylogenetic relationship of Biospeedotrema and Zdzitowieckitrema Sokolov, Lebedeva, Gordeev, & Khasanov, 2019 with respect to the Opecoelidae is currently uncertain and, within the Xiphidiata, these genera are currently without adequate familial classification. However, Caudotestis belongs to the Stenakrinae, a subfamily within the Opecoelidae. Three other stenakrine species—Holsworthotrema enboubalichthys Martin, Huston, Cutmore, & Cribb, 2018, Holsworthotrema chaoderma Martin, Huston, Cutmore, & Cribb, 2018, and Scorpidotrema longistipes Aken’Ova & Cribb, 2003—are integrated into a large clade of the opecoelid trematodes. Therefore, the Stenakrinae is apparently polyphyletic.

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